Because of their long life and good efficacy, mercury vapor lamps have been used commercially for many years, in spite of the bluish-green light they emitted. The subsequent addition of metal halides particularly sodium halide to the arc tube fill ingredients improved efficiency and produced white light. An example of such a device is the metal arc lamp shown in Koury et al. U.S. Pat. No. 3,407,327 issued Oct. 22, 1968. Because of the improvement in luminous efficacy and color rendition, such lamps have become commercially successful. Accordingly, sodium is an important constituent in the chemical fill ingredients of metal halide lamps since it serves to enhance the red spectral region, lowers the color temperature as well as acting as a so called "arc-fastener" thereby increasing the radiating volume and establishing a more stable arc. Clearly, then the loss of sodium leads to degraded lamp performance and is undesirable. The loss of sodium atoms by the movement of Na+ ions through the hot silica of the walls in sodium-containing lamps is well-known. The loss of sodium atoms from NaI frees iodine which can then combine with the mercury in the arc tube to form HgI.sub.2, which leads to difficulties in starting and changes in the color of the emitted radiation. Reference may be made to the textbook entitled "Electric Discharge Lamps" by Dr. John Waymouth, M.I.T. Press 1971, (Chapter 10), for a detailed description of the sodium loss process in metal iodine arc lamps. One solution to the problem, which has been adopted by the major lamp manufactures in the United States, has been the so-called "frameless" harness as taught in Gungle et al., U.S. Pat. No. 3,424,935, 1969, "Harness Construction for Metal Arc-Type Lamp". There is evidence that most of the sodium loss is due to a negative charge on the arc tube walls caused by photoelectric emission from the frame side rods used to support the arc tube within the outer bulb in prior art construction. In the "frameless" construction, there are no side rods running alongside the arc tube and the current return wire for the outer end electrode is a fine piece of tungsten wire, sometimes known as the flying lead, spaced as far away from the arc tube as possible and hugging the curve or bulge in the outer bulb. While such "frameless" construction provides an improvement, lamp designs having a heat conserving quartz sleeve and required rigid supporting structure, preclude such frameless construction.
In other metal halide lamp designs, an inert gas fill is introduced within the outer glass jacket or bulb within which are mounted the arc tube and supporting framework to retard the photoelectric currents and reduce sodium loss. However, heat losses due to convection and conduction reduce the efficacy of such inert gas filled lamps which is particularly objectionable in low wattage designs.
Generally, metal halide discharge lamps are of the intermediate or relatively high wattage variety such as about 175 to 1500 watts for example. Also, it is known that the efficacy of the lumen output to input power decreases as the wattage of the lamp decreases. Thus, it has been generally presupposed that at lower wattages, wattages of 100 watts or less, metal halide discharge lamps would be entirely unsatisfactory in so far as efficacy is concerned. In Bechard et al. U.S. Pat. No. 4,281,274 issued July 28, 1981, there is disclosed a jacketed metal halide discharge lamp, combining a miniature arc tube containing sodium iodide and a standby filament within an outer envelop, with a glass sleeve around the arc tube. The glass sleeve is connected to a point of potential which is positive relative to the arc tube, suitably the anode when the arc is operated on D.C. The glass sleeve appears to prevent sodium loss from the arc by trapping ultraviolet light and by shielding the arc from photoelectrons. The space within the outer envelope or bulb is filled with an inactive gas under pressure. Thus, a glass cylinder and a gas filled outer envelope are utilized to reduce the heat loss due to convection currents. However, structures having gas filled envelopes and accompanying convection currents leave something to be desired in reduction of heat loss in so far as relatively high pressure lamps are concerned.
An improvement in such inert gas filled designs providing low wattage is disclosed in the co-pending application entitled: "LOW WATTAGE METAL HALIDE DISCHARGE LAMP" filed Oct. 29, 1984, bearing U.S. Ser. No. 665,471 and assigned to the Assignee of the present application. As will be described in more detail hereinafter, the low wattage metal halide discharge lamp disclosed in said co-pending application includes an evacuated envelope wherein is disposed a heat reducing member having an arc tube therein. The heat reducing member and the arc tube have a metal band and an outer strap member adjacent one another and adjacent an electrode. The metal band, strap member and electrode are all electrically connected to an electrical lead of one polarity whereby sodium losses from the arc tube are reduced. While such design provided an improvement by reducing sodium loss considerably so that the color temperature shift was decreased from 500.degree. K per 1000 hours to 200.degree. per 1000 hours, further improvement was deemed necessary for a commercially successful product.